Keep them separated until they are about the same size. Bigger birds always pick on the little guys. Then introduce them on neutral territory, like when they are free ranging once they are the same size.
I understand your problem and I tried everything too. When I let them free range they still stayed separate. However, I followed advise that I read on BYC and here's what I did. I put a small pen in my large run and put the young hen in there for about three days. When I fed the hens I always fed next to the pen so they had to be near each other. Then, on the third night I took the young hen out of the pen and put her in the coop with the others. They still pecked at her but not as bad. It's not as bad as before. They are working it out.